3.6 Male & Female Perineum and Reproductive Organs

Perineal Boundaries and Regions

Quick Answer: The perineum is a diamond-shaped region whose corners are the pubic symphysis (anterior), ischial tuberosities (lateral), and coccyx (posterior). A transverse line between the two ischial tuberosities divides it into an anterior urogenital triangle and a posterior anal triangle.

Urogenital Triangle

Contains:

• External genitalia — penis and scrotum in males; vulva (mons pubis, labia majora and minora, clitoris, vestibule) in females.
• Urethra (and in females, the vagina) traversing the urogenital hiatus of the levator ani.
• Superficial and deep perineal pouches (see below).
• Perineal membrane (formerly inferior fascia of urogenital diaphragm) — a fibrous sheet spanning the ischiopubic rami, pierced by the urethra (and vagina).

Anal Triangle

Contains the anal canal surrounded by the external anal sphincter (skeletal muscle, pudendal nerve) and the paired ischioanal (ischiorectal) fossae — wedge-shaped, fat-filled spaces that allow anal canal expansion. The pudendal (Alcock's) canal runs along the lateral wall of each fossa on the obturator internus fascia, transmitting the pudendal nerve and internal pudendal vessels. Ischioanal abscesses can extend across the midline posteriorly to form a horseshoe abscess.

Superficial and Deep Perineal Pouches

Colles fascia is continuous with the Scarpa fascia of the anterior abdominal wall; a rupture of the penile urethra allows extravasated urine to spread within the superficial perineal pouch and ascend the abdominal wall beneath Scarpa fascia, producing perineal and lower-abdominal swelling — the classic extravasation of urine scenario.

Erectile Tissues and External Genitalia

Male Erectile Tissues and External Genitalia

• Corpora cavernosa (paired) — dorsolateral erectile bodies attached to the ischiopubic rami as the crura; covered by ischiocavernosus muscle.
• Corpus spongiosum (single, midline) — ventral, contains the penile (spongy) urethra; expands distally as the glans penis; its proximal bulb is covered by bulbospongiosus muscle.
• Penis — root (crura + bulb) and body (corpora in tunica albuginea); the deep fascia (Buck fascia) binds the corpora; the superficial fascia (Dartos) contains smooth muscle.
• Scrotum — cutaneous sac with dartos fascia (smooth muscle, thermoregulation); the cremaster muscle (skeletal, from internal oblique, supplied by the genitofemoral nerve) draws the testes up.
• Spermatic cord — transmits the ductus deferens, testicular artery (from aorta), cremasteric artery, artery of the ductus deferens, pampiniform plexus of veins, genital branch of the genitofemoral nerve, and sympathetic autonomic fibers; traverses the inguinal canal.

Female Erectile Tissues and External Genitalia (Vulva)

• Mons pubis — fatty eminence over the pubic symphysis.
• Labia majora — fatty folds (homologous to the male scrotum).
• Labia minora — hairless, fat-free folds (no homologue); contain erectile tissue and sebaceous glands; meet anteriorly as the frenulum of the clitoris and posteriorly as the frenulum of the labia (fourchette).
• Clitoris — composed of two crura (corpora cavernosa homologues) and a glans clitoris (corpus spongiosum homologue); covered by the prepuce (hood) formed by the labia minora.
• Vestibule — the cavity between the labia minora containing the urethral orifice, vaginal orifice, and openings of the greater vestibular (Bartholin) glands (homologous to the male bulbourethral/Cowper glands).
• Vestibular bulbs — paired masses of erectile tissue deep to the labia minora, covered by bulbospongiosus.

Reproductive Organ Continuity with the Pelvis

The perineal reproductive structures are continuous with the pelvic viscera:

• Male: penile urethra → membranous urethra (deep pouch) → prostatic urethra → bladder; ductus deferens in the spermatic cord → inguinal canal → pelvis → ejaculatory duct → prostatic urethra.
• Female: vaginal orifice (vestibule) → vagina → cervix → uterus → uterine tubes → ovaries; the round ligament of the uterus traverses the inguinal canal to the labia majora.

Innervation and Vascular Supply of the Perineum

Pudendal Nerve

The pudendal nerve (S2–S4 ventral rami) is the principal nerve of the perineum. After exiting the greater sciatic foramen below piriformis, it hooks around the sacrospinous ligament near the ischial spine (a landmark for pudendal nerve block) and enters the perineum through the lesser sciatic foramen and pudendal (Alcock's) canal. Its branches:

• Inferior rectal nerve — external anal sphincter and perianal skin.
• Perineal nerve — superficial branch to scrotal/labial skin; deep branch to muscles of the deep pouch and external urethral sphincter.
• Dorsal nerve of the penis/clitoris — sensory to the glans and erectile tissue.

Arterial Supply

The internal pudendal artery (branch of internal iliac) accompanies the pudendal nerve in Alcock's canal and gives off:

• Inferior rectal artery — to anal canal below pectinate line and external sphincter.
• Perineal artery — to superficial perineal pouch.
• Dorsal artery of penis/clitoris and deep artery of penis/clitoris — erectile blood supply; the deep artery supplies the corpora cavernosa and is the principal vessel for erection.

Erection, Emission, and Ejaculation

• Erection — parasympathetic pelvic splanchnic nerves (S2–S4) release nitric oxide causing relaxation of cavernous smooth muscle and increased arterial inflow; the corpora fill, compressing venous outflow against the tunica albuginea.
• Emission — sympathetic hypogastric nerves (T11–L2) cause contraction of the vas deferens, seminal vesicles, and prostate, depositing semen into the prostatic urethra.
• Ejaculation — somatic pudendal nerve causes rhythmic contraction of bulbospongiosus, expelling semen through the penile urethra. Simultaneous sympathetic contraction of the internal urethral sphincter prevents retrograde ejaculation.

The pudendal nerve block (landmark at the ischial spine, guided by transvaginal ultrasound) is used for second-stage labor analgesia and perineal repair. It anesthetizes the perineum but not the pelvic viscera, which require autonomic blockade.